Individual-Based Models

Individual-based models are simulations based on the global
consequences of local interactions of members of a population. These
individuals might represent plants and animals in ecosystems,
vehicles in traffic, people in crowds, or autonomous characters in animation
and games. These models typically consist of an environment or
framework in which the interactions occur and some number of individuals
defined in terms of their behaviors (procedural rules) and
characteristic parameters. In an individual-based model, the
characteristics of each individual are tracked through time. This stands in
contrast to modeling techniques where the characteristics of the population
are averaged together and the model attempts to simulate changes in these
averaged characteristics for the whole population. Individual-based models
are also known as entity or agent based models, and as
individual/entity/agent-based simulations.

Some individual-based models are also spatially explicit meaning that
the individuals are associated with a location in geometrical space. Some
spatially explicit individual-based models also exhibit mobility,
where the individuals can move around their environment. This would be a
natural model, for example, of an animal in an ecological simulation.
Whereas plants in the same simulation would not be mobile. Some
individual-based models are not spatially explicit, for example a simulation
of a computer network might be based on individual models of the networked
computers, but their location would be irrelevant. Spatially explicit
models may use either continuous (real valued) or discrete
(integer valued, grid-like) space.

Individual-based models are a subset of multi-agent systems which
includes any computational system whose design is fundamentally composed of
a collection of interacting parts. For example an "expert system" might be
composed of many distinct bits of advice which interact to produce a
solution. Individual-based models are distinguished by the fact that each
"agent" corresponds to autonomous individual in the simulated domain.

There is an overlap between individual-based models and
cellular automata.
Certainly cellular automata are similar to spatially-explicit, grid-based,
immobile individual-based models. However CAs are always homogeneous and
dense (all cells are identical), whereas a grid-based individual-based
model might occupy only a few grid cells, and more than one distinct type
of individual might live on the same grid. (Of course a CA can have cells
in various states, and so represent concepts like empty or
occupied by type 3. Perhaps the significant difference is whether
the simulation's inner loop proceeds cell by cell, or individual by
individual. (Although that distinction is muddied by parallel-processing
hardware.)) The philosophical issue is whether the simulation is based on a
dense and uniform dissection of the space (as in a CA), or based on
specific individuals distributed within the space.

Of course, note that everyone uses terminology differently, so take the
definitions above with a grain of salt. ("Your mileage may differ.")

My interest in this area began when I made a model of
bird flocks and related group motion. As a result I am particularly
interested in individual-based models using spatially explicit mobile agents
in continuous space. This bias may be reflected in the selection of
resources listed below.

Online resources

These are general purpose software toolkits useful for implementing
individual-based models.

The Weaver
Project by
Matt
Hare, Alan Sibbald, and Alistair Law, is a spatially explicit,
individual-based model of the
red
grouse in Scotland's heather moorland. It seeks to provide wildlife
managers with advice on appropriate strategies to restore grouse
populations.

Carl Anderson uses agent-based
models to study the organisation of workers and work in insect societies
and related regulatory mechanisms, particularly task partitioning
and self organization in ant and bee colonies. Some papers are
avaialble at his
older web site.

Arborgames by
Melissa Savage and
Manor Askenazi
examines the role of forest fire on species diversity. Local interaction
of trees in a neighborhood allows the model to generate complex landscape
dynamics. Bruce Sawhill is collaborating on analysis of model results.
Robert Bell
is working on a application in Yellowstone.

Theoretical Ecology of Spatial Heterogeneity: An IBM Approach
ongoing work by Kim
Cuddington on "...the effects of limited mobility and spatial
structure or heterogeneity on the population dynamics and stability of
communities." See also these resources on
theoretical ecology.

Aspen, a microanalytic model to simulate the U.S. economy. Aspen
uses economic agents to represent the various decision-making segments,
and the microanalytic simulation process models each agent individually.
See also this earlier
press briefing.

Transportation Analysis and Simulation System at the
Los Alamos National Laboratory.
"TRANSIMS models a metropolitan region with a representation of the
inhabitants, their activities, and the transportation infrastructure.
TRANSIMS then simulates the movement of individuals across the
transportation network, including their use of vehicles such as cars or
buses, on a second-by-second basis."

The STEER Traffic
Simulator (Signals/Traffic Emulator with Event-based Responsiveness)
is a program intended to simulate traffic on an urban network, modeling
up to tens of thousands of vehicles.

METROPOLIS 1.0 is a
modular system for Dynamic Traffic Simulations: It is aimed towards
on-line as well as off-line simulations of traffic flows in an urban
context and for large networks.

SmartPath simulation and animation package for traffic studies.
See also Smart AHS
a specification, simulation and evaluation framework for modeling,
control and evaluation of Automated Highway Systems (AHS). Both part of
California's PATH
(Partners for Advances Transit and Highways)

The Amorphous
Computing group at MIT explores issues such as "How do we obtain
coherent behavior from the cooperation of large numbers of unreliable
parts that are interconnected in unknown, irregular, and time-varying
ways?"

Project for the Simulation of Social Behaviour within the Italian
National Research Council - Institute of Psychology. Within this
program, formal models of interactions among intelligent autonomous
agents have been developed and some computer simulation studies have
been conducted.